The recent trend in printing form plates for newspaper, magazines and the like is to replace conventional lead form plates by photosensitive resin form plates which can be quickly processed in a simple manner and eliminate the risk of contamination with toxic substances as typified by heavy metals. The photosensitive resin form plates are produced by processing liquid and solid photosensitive resins which undergo a chemical change upon exposure to ultraviolet light or the like, wherein the liquid photosensitive resin in exposed areas cures and conversely, the solid photosensitive resin in exposed areas decomposes into liquid. This process includes a wash-out step in which photosensitive resin plates after exposure are set in a wash-out apparatus where the uncured portions or the portions liquefied by decomposition are washed out by injecting a developer for the photosensitive resin by means of a pump or the like.
The developers used in the wash-out step are required to have intense washing action and are generally aqueous solutions containing an inorganic alkali salt such as sodium borate and a surface active agent. Therefore, the developers tend to foam due to injection by a pump in the wash-out step, which results in bubbles emerging out of the apparatus or induces cavitation in the pump to reduce its injection pressure, both interfering with processing and reducing operation efficiency. It is thus a common practice for such a step to previously add defoamers to the developers to prevent foaming.
Among these defoamers, emulsion type defoamers having an oil compound consisting of dimethylpolysiloxane and silica fine powder dispersed in water with a surface active agent are widely used. These defoamers, however, have a drawback that emulsion is broken and in some cases, they rather promote foaming when a solution to which the defoamer is added contains an inorganic alkali salt and/or when high shearing forces are applied for a prolonged period of time as by injection through a pump. As a result of emulsion breakage, the oil compound will agglomerate and form a scum which will adhere to the apparatus and form plates. The water-repellent dimethylpolysiloxane can impart repellency to the form plates, detracting from the quality thereof.
One solution to this problem is to use an organopolysiloxane modified with a polyoxyalkylene group as disclosed in U.S. Pat. No. 3,233,986, Japanese Patent Publication No. 58126/1983, and Japanese Patent Application Kokai No. 180407/1982. This defoamer, however, undesirably has a low defoaming power. Attempts have been made to increase defoaming power by using such an organopolysiloxane modified with a polyoxyalkylene group along with an oil compound consisting of a diorganopolysiloxane, typically dimethylpolysiloxane and silica fine powder as disclosed in Japanese Patent Application Kokai No. 34854/1978, 161850/1980, 129013/1981, and 189933/1984, and Japanese Patent Publication No. 34847/1986, 23084/1980, 22638/1977, and 30872/1989.
However, some problems arise if a developer for photosensitive resin having such a defoamer added thereto is used in practice. If a larger amount of the oil compound component consisting of a diorganopolysiloxane and silica fine powder is blended in the composition in order to retain defoaming ability over a prolonged period of time, then the hydrophobic oil compound component undergoes agglomeration in the wash-out step where mechanically intense shearing forces are applied for a long period of time, adhering to the apparatus and form plates which are contaminated and rendered repellent. In turn, if the amount of the oil compound component is reduced, it is difficult to retain defoaming ability over a prolonged period of time.
An object of the present invention is to eliminate the above-mentioned problems. Another object is to provide a defoamer composition which can retain defoaming ability over a prolonged period of time. A further object is to provide a defoamer composition which, when added to solutions to be used under severe conditions where high shearing forces are applied for a long time as in the wash-out step for a photosensitive resin after exposure, can maintain minimal foamability, has high enough mechanical stability to prevent agglomeration, and thus eliminates any interference to or contamination of the associated apparatus and form plates.